Method of growing quartz crystals and seed plate therefor

ABSTRACT

There is disclosed a method of growing monocrystalline quartz by use of a Z-plate seed. The Z-plate seed has facet inhibiting projections on certain edges to attenuate formation of major rhombohedral facets which limit the amount of useful material produced. The seed plate is of a generally rectangular shape having its length to width ratio of 10:1 or less.

US. Patent Nov. 4, 1975 Sheet 1 of2 3,917,506

US. Patent Nov. 4, 1975 Sheet 2 of2 3,917,506

BACKGROUND OF THE INVENTION This invention relates to the growing ofmonocrystalline material and more particularly to the growing of quartzcrystals.

The use of quartz crystals in piezoelectric oscillators and crystalfilters led to a demand for monocrystalline quartz in relatively largequantities. Since quartz crystals of the natural occurring type arerelatively rare and have non-uniform characteristics there has developeda new industry relating to the manufacture of monocrystalline quartzmaterial to serve the electronics industry.

In the manufacture of quartz crystals a plurality of seed crystals areplaced in the upper portion of a vertical autoclave. By maintaining thesupply of silica and the seed crystals at a relatively high temperatureand pressure for a period of time while maintaining a temperaturedifferential between the supply chamber and the seed chamber, silicafrom the supply portion of the autoclave will be deposited upon theseeds thereby pro ducing monocrystalline quartz.

Quartz crystals, like most crystals, exhibit a preferential growthcharacteristic, that is growth takes place more rapidly on certaincrystallographic surfaces as contrasted with other crystallographicsurfaces of the seed. Thus, more rapid deposition takes place on a Z-surface (a surface perpendicular to the Z-axis of the quartz crystal) ascontrasted with deposition on all other surfaces. Thus, a seed havingits major surfaces perpendicular to the Z-axis of the quartz crystal ismuch preferred. A seed of this type is referred to as a Z-plate.

Certain crystallographic surfaces of a quartz crystal exhibit extremelylow deposition or growth rates. In some cases, this deposition rate isvirtually negligible such as to provide certain surfaces which may bereferred to as growth inhibiting surfaces. For example. the naturalprismatic faces of a quartz crystal (Y- plate" surfaces) exhibitvirtually no growth during the deposition process. The X-axis of thequartz crystal is its polar axis; that is, the axis which exhibitselectrical asymmetry. When a quartz crystal is tensioned along itsX-axis a difference in electrical potential appears. The end at whichthe positive charge appears is designated the +X direction. It has beenfound that growth on faces perpendicular to the X-axis (X-platesurfaces) in the X direction is approximately /2 of the growth rate inthe Z-direction, while growth proceeds even more slowly in the +Xdirection. Growth in the direction of the first order and second orderrhombohedron faces is similarly relatively slow.

From the foregoing, it will thus be seen that a seed should be sodimensioned as to limit development of the prism and rhombohedral facesand not depend on growth in these directions or in the X-dircctions.

One of the more common crystal cuts used for oscillator plates is the ATcut. The AT cut has its major sur faces parallel to the X-axis and at anangle of 35 l4 to the Z-axis. Thus, the quartz crystal produced by acrystal growing process should be grown so as to yield the maximumnumber of AT cut plates or blanks. Many of the other cuts suitable foroscillator and filter plates,

the BT, CT, etc. cuts, also have major surfaces parallel to the X-axisand are generally referred to collectively as X-cuts. Thus, a crystalresulting from a crystal growing process having an orientation such thatcuts parallel to the X-axis may be readily made is highly desirable.

SUMMARY OF THE INVENTION It is an object of this invention to provide animproved method and seed plate for the growth of monoerystalline quartz.

Another object of the invention is to provide an improved method andseed plate for the growing of quartz wherein the growth is primarily ina single direction relative to its crystallographic axis.

A still further object of the invention is to provide a method and seedplate for growing of monocrystalline quartz wherein the growth isprimarily in the Z-axis direction.

Another object of this invention is to provide a seed plate shaped toinhibit growth of limiting rhombohedron faces and in the X-axisdirections.

A still further object of the invention is to provide a method and seedcrystal for the growing of monocrys talline quartz in a rapid andeconomic manner to produce monocrystalline quartz giving an optimizedyield for oscillator and filter plates.

In accordance with these objects there is provided a seed for growingmonocrystalline quartz from a supersaturated medium which comprises aplate of monocrystalline quartz having its major generally rectangularsurfaces perpendicular to the Zaxis, its minor surfaces perpendicular torespective X- and Y-axes and a facet inhibiting face on at least one ofthe surfaces parallel to the said Y-axis, and method utilizing theaforesaid seed plate.

THE DRAWINGS Further objects and advantages of the invention will beunderstood from the following complete description thereof and from thedrawings wherein:

FIG. 1 is a pictorial representation of a quartz crystal depicting itsmajor crystallographic faces;

FIG. 2 is a cross section thereof depicting a seed plate crystal inaccordance with an embodiment of the invention;

FIG. 3 is a perspective view of a seed plate;

FIG. 4 is another embodiment of a seed plate crystal in accordance withthe invention; and

FIG. 5 is a crystal resultant from the use of the seed plate of FIG. 3.

DETAILED DESCRIPTION As shown in FIG. I a quartz crystal 10 inaccordance with conventional trigonal nomenclature has a Z-axisextending parallel to its major hexagonal prism faces 11, a Y-axisperpendicular to the Z-axis and to the major prism faces II and anX-axis orthogonal therewith. The )(axis therefore passes through theintersection between adjacent major prism faces II. A quartz crystalnormally exhibits major rhombohedral faces 12 and minor rhombohedralfaces 13. Trigonal bipyramidal faces, trigonal trapezohedral faces andother faces sometimes exhibited by natural quartz are not pertinent tothe present invention and are not shown. Also. no distinction need bemade between left-handed and right-handed quartz since growth occurs inthe same sense as the seed. The quartz crystal is sometimes hexagonallydescribed in terms of its Z-axis and the three axes through theintersection of the major prism faces (X,, X X However, in theelectronics field the orthogonal X. Y, Z nomenclature is more commonwith the Z-axis being the axis of crystal symmetry and the X-axis beingthe polar axis, which nomenclature is used herein.

In accordance with the invention a seed plate 16 is oriented withrespect to the crystallographic axes as shown in FIG. 2. The seed plate16 has end edge surfaces l7 perpendicular to the Yaxis, hence having itsmajor length in the Y dircction. The side edge surfaces 18 parallel theY-axis, with its minor length in the X direction. The thickness of theplate is in the Z- direction, hence providing a seed plate having itsmajor surfaces 19 perpendicular to the Z-axis for primary growth in thedirection of the Zaxis. Preferable dimensions for a Z-plate inaccordance with the invention is one having a thickness of 0.080 of aninch, a length of approximately 6 inches along the Y-axis and a width ofapproximately 2 inches in the X-direction. Since growth in theXdirection is not desired, the Y to X ratio should be less than :1 andis preferably about 3:1.

in accordance with the invention the crystals growth is enhanced by anedge 18 of the seed plate 16 configurated to result in triangularprojections 20 (HO. 3) adjacent ends 17. These projections result in anincrease in usable quartz by effecting an inhibition of the formation ofrhombohedral faces which faces limit growth and forcshorten theresultant quartz crystal seed plate. End tabs 21 are useful for mountingthe seed plates in the autoclave. They may be eliminated if desired.providing some other suitable holding means is provided,

thus effectively increasing the overall length of the seed plate forgrowth of "2 material. 2" material. as used herein, is meant to be thatmaterial which is basically developed on a surface normal to the Z-axis.Z material is the most desirable material from both electrical andproduction standpoints because this material has more consistentelectrical characteristics and grows most rapidly.

A seed plate I6 is depicted in FIG. 5 with a resultant mono-crystallinequartz crystal superimposed therearound. The crystal 30 is an elongatedbar with the primary growth surfaces 3l and 32 perpendicular to theZ-axis having a generally flat surface which would be termed a basalpinacoid since it is parallel to the XY plane. However, these faces areof the non-dominant type and have a rippled surface. The top surface 33and the bottom surface 34 lie in the YZ planes and are in verse prismsurfaces, i.e., surfaces parallel to the Y-axis rather thanperpendicular thereto as in a natural crystal. Thus, the manufacturedcrystal has an overall shape in which the Y-axis appears to be the axisof crystal symmetry for a tetragonal class crystal. Hexagonal prismfaces I la appear at the ends of the crystal. However, two of theseprism faces are restricted by the fact that the seed crystal was held atthese end faces during growth and the other four prism faces do notappear or are restricted by the minor amount of growth in the X-dircction. The latter four prism faces if they appear, lie in planesthat are at 60 to the X2 plane. The other facets at the end of the grownbar are the low and high angle major rhombohedral faces 12a and 12b andthe high angle minor rhombohedral faces 13b. Certain minor faces 35 and36 appear which are parallel to the Y-axis and at an angle to the X andZ axis along the surface 33. It will be thus seen that much of the endof the bar structure is terminated by the high angle rhombohedral faces12b and 13b which lie in planes that are at approximately 30 to the X2plane rather than being dominated by the low angle rhombohedral faceswhich are at angles of approximately 60 to the X2 planes. There is thusmore useful quartz in a bar grown in accordance with the invention thanin the prior art wherein the low angle major rhombohedral faces greatlyforeshortened the end structure of the monocrystalline quartz crystal.By so controlling growth in the X-direction at least 25% more usablequartz length may be obtained in a crystal of practical length.

The dimensions of the seed plate are selectable based on variouscriteria, such as growth rate, end use and electrical characteristics.The growth rate is dependent on the development of growth limiting facessuch as the prismatic faces, the major rhombohedral faces and the minorrhombohedral faces. Because of the fast growth rate in the Z-direction,the plate may be as thin as may be practically cut so that a maximumnumber of seed plates may be obtained from any given stock. The width inthe X-direction must be sufficient for development of the high anglerhombohedral faces 12b and 13b which lie in planes at 30 to the X2 planeand intersect the Z-plate surfaces in a line parallel to the X-axis sothat minimized foreshortening of the bar results. The length in theYdirection should be sufficient for practical handling for cuttingpurposes.

The overall size is determined by the amount of 2- material that may begrown in a practical amount of time, for example, 60-75 days. Withinthis period of time, approximately one-half inch of growth occurs inboth Z-directions, hence, yielding a bar approximately one inch thick inthe Z-direction. Since the rhombohedral surfaces 120 intersecting the Xand Y axes of the crystals are limiting, the seed should be at least twoinches in one of these directions to obtain the basal pinacoids 31 and32 and inverse prisms 33 and 34. Since the Y-direction is greatlyaffected by the low angle limiting rhombohedral faces 120, it ispreferable that the seed have the two inch dimension in the Y-axis.However, so long as the dimension in the X-axis of the seed is greaterthan two inches, a shorter length in the Y- direction may be consideredsince the sides would be dominated by the high angle rhombohedral faces12b and [3b. The crystal must be long enough in the X- direction toinsure formation of the high angle rhombohedral faces l2b and 13b whichform intersections with the Z-plate surfaces parallel to the X-axis.These will appear at the four ends of the Z-plate surfaces provided theseed plate is at least one inch in the X-direction, two of these facesbeing major rhombohedral faces 12!) and two of the faces being minorrhombohedral faces 13!). These minor rhombohedral faces will notnecessarily appear in a regular rectangular plate without the use of theX-growth inhibition projection 20 unless the plate is approximately oneinch and one-half in the X- direction. With the use of X-growthinhibiting configuration the formation of these rhombohedral faces 12band 13b is enhanced and the X-direction dimension need be only one inch.

Another criteria for the dimensions of the seed plate in accordance withthe invention is the end use, i.e., the oscillator or filter plates tobe cut therefrom. Many of the cuts suitable for oscillator and filterplates, the AT,

BT, CT, etc. cuts. have major surfaces parallel to the X-axis.Therefore, it is highly desirable to use a crystal seed which results ina bar of quartz crystal which has its width parallel to the X-axis andhence perpendicular to the Y-axis, therefore its length should extend inthe Y-axis direction. Since most of the cuts parallel to the X-axis areat some angle to the Z-axis. it is preferable that the length of theplate be made as long practical in the Y direction to increasepercentage yield. For example, an AT plate is at 35 to 14' to theZ-axis. Because of this angle. the effective length of bar in the Y-direction which produces usable quartz is proportional to the tangentofthe angle, or approximately 7/10 of an inch per inch of thickness inthe Z-direction. Thus. if one assumes a 1 inch by 1 inch rectangular barof quartz (ignoring for the moment the faccting at the ends of asynthesized monocrystalline bar) its Y-axis length must be more than7/10 of an inch to produce any useful quartz. Since the losses at theends are fixed. yield increases with length, cg. a 2-inch bar wouldproduce approximately a 50% yield, a 3-inch bar approximately 66'72. a4inch bar 75%. etc.

Another criteria for the dimensions of the seed plate is the electricalcharacteristics of the resultant quartz material. If growth is permittedin the X-direction. the seed crystal contains what might be termedX-material and Z-material. Thus. certain blanks cut from a bar grown inthis manner will contain X-material and Z material with an interfacetherebetwecn. The electrical characteristics of the X-matcrial may notbe the same as the characteristics of the [material and the interfacemakes an electrical discontinuity. Blanks with this interface areunsuitable for oscillator or filter plates. Therefore. the dimension ofthe seed in the X-direction should be at least one blank width. Forexample. if /2 inch blanks are to be formed, the dimension should be atleast /2 inch, or a multiple thereof, for example. one or two inches.With all of the dimensional criteria taken into account, it ispreferable that the seed plate 16 have a length to width ratio of lessthan 10. and preferably approximately 3.

The facet inhibiting projections need be provided only on the -X edge ofseed plate to attenuate the two largest low angle rhombohedral surfaces12a. However. some further improvement of effective length is attainedby recessing of the +X edge 18 as well. A seed plate of this type isshown in FIG. 4. The seed plate 16' has end edges 17' and side edges 18configurated to provide projections 20' in both of the X-directions. Thefour projections 20' will attenuate all four limiting rhombohedrons 12a.While the invention hsa been disclosed by way of the preferredembodiments thereof. it will be apparent to one skilled in the art thatsuitable changes may be made therein without departing from the spiritand scope of the invention.

What is claimed is:

I. A method of growing monocrystalline quartz by the transfer ofquartzfrom an aqueous growing solution onto a crystal seed which comprises thesteps of provid ing an elongated quartz crystal seed plate having itslength extending substantially parallel to a crystallographic Y-axis andits thickness extending parallel to the Z-axis, forming said plate toprovide a pair of spaced apart integral projections having a generallytriangular portion on a side parallel to the crystallographic Y axis andextending outwardly from said side in the crystallographic X-directionwith one side of the triangular portion forming an obtuse angle with theside. each projection being positioned adjacent an end of said side withsaid side extending substantially continuously thercbetwccn. andexposing said seed plate under growing conditions at elevatedtemperature and pressure to an aqueous solution in contact with a supplyof silica to affect the transfer of quartz onto the seed bar.

2. A seed for growing monocrystalline quartz comprising a plate ofmonocrystalline quartz material having its thickness extending generallyparallel to the Z- axis and major generally rectangular surfacesperpendicular to the Z-axis, the longitudinal sides of said plateextending in a crystallographic Y-axis direction. and the transverseends of said plate extending in the crystallographic X-axis direction;and a pair of spaced apart integral projections having a generallytriangular portion on at least one ofthe sides of said plate parallel tothe Y-axis and projecting outwardly from the one side in the X-axisdirection with one side of the triangular portion forming an obtuseangle with the side. each projection being positioned adjacent an end ofthe one side with the one side extending substantially continu ouslytherebetwcen to provide facet inhibiting charac teristics when said seedplate is placed in a crystalgrowing environment.

3. A seed as recited in claim 2 and further including a pair ofprojections on the other side parallel to the Y- axis.

4. A seed recited in claim 3 wherein the length to width ratio of saidseed is less than 10:].

S. A seed as recited in claim 4 wherein said length to width ratio isapproximately 3:].

6. A seed as recited in claim 2 and further including support tabsintegrally formed at the ends of said seed plate.

7. A seed for growing monocrystalline quartz comprising a plate ofmonocrystalline quartz material having major generally rectangularsurfaces perpendicular to its Z-axis and its width extending in adirection parallel to a major surface of a standard oscillator cut andlongitudinal edges ofthc plate extending perpendicular to its widthdirection and a pair of spaced apart facet inhibiting integralprojections having a generally triangular portion extending from one ofthe longitudinal edges in the width direction with one side of thetriangular portion forming an obtuse angle with the edge. eachprojection being positioned adjacent an end thereof.

8. A seed for growing monocrystallinc quartz as recited in claim 7wherein said standard oscillator cut is an AT cut.

9. A seed for growing monocrystalline quartz as recited in claim 8 andfurther including a pair of projections extending from its edges in theother width direction.

10. A seed as recited in claim 9 and further including support tabsintegrally formed at the ends of said seed.

11. A seed for growing monocrystalline quartz as re cited in claim 8wherein the length to width ratio ofsaid seed plate is less than 10:1.

12. A seed for growing monocrystallinc quartz as recited in claim 1]wherein said length to width ratio is approximately 3:].

1. A METHOD OF GROWING MONOCRYSTALLINE QUARTZ BY THE TRANSFER OF QUARTZFROM AN AQUEOUS GROWING SOLUTION ONTO A CRYSTAL SEED WHICH COMPRISES THESTEPS OF PROVIDING AN ELONGATED QUARTZ CRYSTAL SEED PLATE HAVING ITSLENGHT EXTENDING SUBSTANTIALLY PARALLEL TO A CRYSTALLOGRAPHIC Y-AXIS ANDITS THICKNESS EXTENDING PARALLEL TO THE Z-AXIS, FORMING SAID PLATE TOPROVIDE A PAIR OF SPACED APART INTEGRAL PROJECTIONS HAVING A GENERALLYTRIANGULAR PORTION ON A SIDE PARALLEL TO THE CRYSTALLOGRAPHIC Y-AXIS ANDEXTENDING OUTWARDLY FROM SAID SIDE IN THE CRYSTALLOGRAPHIC -X-DIRECTIONWITH ONE SIDE OF THE TRIANGULAR PORTION FORMING AN OBTUSE ANGLE WITH ONESIDE, EACH PROJECTION BEING POSITIONED ADJACENT AN END OF SAID SIDE WITHSAID SIDE EXTENDING SUBSTANTIALLY CONTINUOUSLY THEREBETWEEN, ANDEXPOSING SAID SEED PLATE UNDER GROWING CONDITIONS AT ELEVATEDTEMPERATURE AND PRESSURE TO AN AQUEOUS SOLUTION IN CONTACT WITH A SUPPLYOF SILICA TO AFFECT THE TRANSFER OF QUARTZ ONTO THE SEED BAR.
 2. A seedfor growing monocrystalline quartz comprising a plate of monocrystallinequartz material having its thickness extending generally parallel to theZ-axis and major generally rectangular surfaces perpendicular to theZ-axis, the longitudinal sides of said plate extending in acrystallographic Y-axis direction, and the transverse ends of said plateextending in the crystallographic X-axis direction; and a pair of spacedapart integral projections having a generally triangular portion on atleast one of the sides of said plate parallel to the Y-axis andprojecting outwardly from the one side in the X-axis direction with oneside of the triangular portion forming an obtuse angle with the side,each projection being positioned adjacent an end of the one side withthe one side extending substantially continuously therebetween toprovide facet inhibiting characteristics when said seed plate is placedin a crystal-growing environment.
 3. A seed as recited in claim 2 andfurther including a pair of projections on the other side parallel tothe Y-axis.
 4. A seed as recited in claim 3 wherein the length to widthratio of said seed is less than 10:1.
 5. A seed as recited in claim 4wherein said length to width ratio is approximately 3:1.
 6. A seed asrecited in claim 2 and further including support tabs integrally formedat the ends of said seed plate.
 7. A seed for growing monocrystallinequartz comprising a plate of monocrystalline quartz material havingmajor generally rectangular surfaces perpendicular to its Z-axis and itswidth extending in a direction parallel to a major surface of a standardoscillator cut and longitudinal edges of the plate extendingperpendicular to its width direction, and a pair of spaced apart facetinhibiting integral projections having a generally triangular portionextending from one of the longitudinal edges in the width direction withone side of the triangular portion forming an obtuse angle with theedge, each projection being positioned adjacent an end thereof.
 8. Aseed for growing monocrystalline quartz as recited in claim 7 whereinsaid standard oscillator cut is an AT cut.
 9. A seed for growingmonocrystalline quartz as recited in claim 8 and further including apair of projections extending from its edges in the other widthdirection.
 10. A seed as recited in claim 9 and further includingsupport tabs integrally formed at the ends of said seed.
 11. A seed forgrowing monocrystalline quartz as recited in claim 8 wherein the lengthto width ratio of said seed plate is less than 10:1.
 12. A seed forgrowing monocrystalline quartz as recited in claim 11 wherein saidlength to width ratio is approximately 3:1.